Biomedical Engineering Reference
In-Depth Information
15.2 DEGRADABLE POLYMERS
Degradable polymers can be either of a natural origin or synthetically manufactured. Biodegradable
polymers can be either natural or synthetic. In general, synthetic polymers offer greater advantages
than natural materials in the way that they can be tailored to impart a wide range of predictable prop-
erties with a greater control on manipulation of physical, mechanical, and degradation properties.
The synthesis of polymers in a laboratory or a manufacturing unit ensures the use of more reliable
sources of raw materials, which eliminates the problems of immunogenicity that may be seen as a
common occurrence in polymers from natural origins. Some examples of synthetic and naturally
occurring polymers that have relevance in the fi eld of biomaterials are presented in Table 15.1.
The essential component in a degradable polymer is the presence of a heteroatom within the
backbone of the polymer. In general, a polymer with a -C-C- backbone is stable and does not tend
to degrade; however, the presence of anhydrides, esters, amide, etc. can confer biodegradable prop-
erties to the polymer. Biodegradability can therefore be engineered into polymers by the judicious
introduction of chemical linkages such as anhydrides, esters and or amide bonds, among others
polyesters, poly(ethylene oxide) (PEO), etc.
15.2.1 P
OLYESTERS
15.2.1.1 Poly(
α
-Hydroxyacids)
Biopolymers are of great interest in the fi eld of biomaterials and their application in healthcare.
However, there are some problems for practical applications such as their poor physical proper-
ties, poor processability, and high production costs. The polymers derived from α-hydroxy acids,
namely, PLA and PGA, (Figure 15.1) have found the most extensive use, primarily as materials for
sutures, dating back to the early 1960s due to their superior biocompatibility and acceptable deg-
radation profi les.
1,2
These polymers remain popular for a variety of reasons including the fact that
both of these materials have properties that allow hydrolytic degradation. Once degraded, natural
pathways remove the degradation products, namely, the monomeric components of each polymer,
glycolic acid that can be converted to other metabolites or eliminated by other mechanisms, and
lactic acid that can be eliminated through the tricarboxylic acid (TCA) cycle.
The group of esters derived from α-hydroxy acids yield polymers such as PGA and PLA,
which are hydrolytically unstable. Degradable polyesters derived from monomers such as lactide,
TABLE 15.1
Some Naturally Occurring and Synthetic Degradable Polymers
Natural Polymers
Synthetic Polymers
Albumin
Poly(lactic acid)
Alginates
Poly(l-lactic acid)
Collagen (proteins)
Poly(d,l-lactic acid)
Chitin, chitosan (polysaccharides)
Poly(glycolic acid)
Fibrin
Poly(ε-caprolactone)
Poly(
p
-dioxanone)
Trimethylene carbonate
Polyanhydrides
Polyorthoester
Polyurethanes
Poly(amino acids)
Polyphosphazenes
